I know things... A lot of things.
Join Date: Feb 2015
Motorcycle: GSXR 750 K4
Re: 06 GSXR 750 Battery drain & R/R
Try checking out a couple of tutorials on using DMMs, just to make sure you don't have any misconceptions. What you need to do is: measure the current through the battery with the ignition off, with the DMM in current mode and bridging the disconnected negative wire with the negative pole. Note that, as Bill mentions not setting the DMM properly can lead to shorts, damage to the DMM, or worse, so make sure you're doing the right thing when the DMM is set in current mode. Consult the manual, then the Internet if unsure, or ask here.
Then you need to measure the voltage across the battery poles, with the engine running at 5000 rpm. If both tests check out, you know a) your battery is being charged and b) it's not being discharged overnight, so it should be ok, unless the battery itself is defective. If a test doesn't check out, you can focus on the relevant section of your electrics.